The invention concerns a voltage-current converter having a first current mirror containing two transistors that are designed such that under identical drive conditions the current flowing through the first transistor is greater than the current flowing through the second transistor, which constitutes the output current of the voltage-current converter, by a predetermined factor.
Voltage-current converters are well-known in the prior art, and are used for converting an input voltage into a proportional output current. This is required, for example, for the voltage-controlled oscillator (VCO) in a phase-locked loop (PLL).
The voltage-current converter that is known in the art and that has been mentioned above is shown in FIG. 2. It contains a current mirror 10 having two normally-off n-channel MOSFETs 12, 14 (metal-oxide-semiconductor field-effect transistors). The current mirror 10 is programmed using a series resistor 16 that is connected in series with the drain of the first transistor 12 to the input voltage UE. The series resistor 16 determines the drain current I12 of the first transistor 12, and this drain current I12 constitutes the input current IE of the current mirror 10.
The gates of the two transistors 12, 14 are connected together and are also connected to the drain of the first transistor 12, so that both transistors 12, 14 are driven under the same conditions. The source of the first transistor 12 is connected to ground. The source of the second transistor 14 is connected to ground, and the output current IA of the voltage-current converter is taken from the drain of the second transistor 14.
The current mirror 10 is disclosed in FIG. 6.21 in the book SEIFART, MANFRED, Analoge Schaltungen-5. Auflage (Analog circuits-5th Edition, Verlag Technik GmbH, Berlin, 1996, DE (ISBN 3-341-01175-7). The circuit shown in FIG. 2 is different from the voltage-current converter that is known from Seifart in that the input voltage UE is connected to the series resistor 16 instead of to the supply voltage UDD. Consequently, the input voltage UE is proportional to the input current IE in accordance with the resistance value of the series resistor 16.
Since the transistors 12, 14 are operated in the saturation region, their respective drain currents I12, I14 are proportional to each other. Provided the remaining parameters, such as the surface mobility of the charge carriers in the channel xcexc0, the gate capacitance per surface area C0x and the threshold voltage UT, are identical for the transistors 12, 14, then this proportionality can be set simply by selecting the geometrical dimensions of the transistors 12, 14. In this case the following equation holds for the two drain currents I12 and I14:
I14/I12=xcex214/xcex212,
where xcex2=W/L is the geometrical quotient of a transistor of channel width W and channel length L.
If the layout of the first transistor 12 and the second transistor 14 on the chip is such that the geometrical dimensions result in the equation xcex212=10xc2x7xcex214, for instance, by the channel of the first transistor 12 being made the same length but ten times wider than the channel of the second transistor 14, then one accordingly obtains the relationship I12=10xc2x7I14.
Thus in this case, because of the aforementioned proportionality between the input voltage UE and the input current IExe2x89xa1I12, the drain current I14 of the second transistor 14, which constitutes the output current IA of the known voltage-current converter, is proportional to the input voltage UE.
Since in the cited applications of the phase-locked loop, the input voltage UE normally lies in the range of 2 to 5 volts, and the required output current intensity IA is meant to lie in the region of a few nanoamps, the series resistor 16 must have a resistance value in the region of several megaohms (Mxcexa9). Resistances of this order of magnitude, however, require a very large area in integrated circuits, which is a major disadvantage because the costs of integrated circuits are mainly determined by the area requirement.
It is accordingly an object of the invention to provide a voltage-current converter which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a voltage-current converter with a first current mirror including a first transistor and a second transistor each being designed such that under identical drive conditions a current flowing through the first transistor is greater than a current flowing through the second transistor by a predetermined factor; a second current mirror including a first transistor and a second transistor; and a MOSFET connected in series with the first transistor of the first current mirror. The MOSFET has a gate connected to an input voltage. The current flowing through the second transistor is an output current of the voltage-current converter. The first transistor of the first current mirror and the first transistor of the second current mirror are connected in series to a supply voltage. The second transistor of the first current mirror and the second transistor of the second current mirror are connected in series to the supply voltage.
In accordance with an added feature of the invention, a current flowing through the first transistor of the second current mirror is equal to a current flowing through the second transistor of the second current mirror.
In accordance with an additional feature of the invention, the first transistor of the first current mirror and the second transistor of the first current mirror are operated in weak inversion.
In accordance with another feature of the invention, the MOSFET has a threshold voltage such that the voltage-current characteristic starts at 0.
In particular, it is an object of the invention to provide a voltage-current converter that requires less area that that required by known voltage-current converters.
In the voltage-current converter, the series resistor 16 previously required in the voltage-current converter known in the art is dispensed with, and since the MOSFET that is now provided occupies a considerably smaller area in an IC compared with a resistor, a considerable area savings is obtained, even though more components are provided compared with the voltage-current converter known in the art.
In order to simplify the explanation of how this voltage-current converter works, it is assumed below that in the second current mirror the two transistors are identical, which here implies that currents of equal magnitude flow through them under identical drive conditions. In addition it is assumed that the factor equals ten.
If the first current mirror were considered on its own, currents of different magnitudes would flow through its two transistors under the same drive conditions, or more precisely the current through the first transistor would equal ten times the current through the second transistor in accordance with the factor. In other words, the first transistor has a conductance that is ten times the conductance of the second transistor in accordance with the factor.
This first current mirror is not on its own, however, but is connected in series with the second current mirror to the supply voltage, which, like the input voltage, lies normally in the range 2 to 5 volts. The two first transistors are connected in series and form the input-current path of the voltage-current converter. The two second transistors are connected in series and form the output-current path of the voltage-current converter. The two identical transistors of the second current mirror ensure that currents of equal magnitude also flow through the two non-identical transistors of the first current mirror. Since this has no effect on their conductances, however, the voltage drop across the first transistor is only one tenth of the voltage drop across the second transistor in accordance with the factor. The remaining voltage, i.e. the difference between these two voltages, falls finally across the MOSFET that is connected in series with the first transistor, and thus constitutes its drain-source voltage.
This drain-source voltage remains constant to a close approximation and equals, for example, 60 mV. This value is selected with regard to the previously mentioned input-voltage range of 2 to 5 volts, and is small enough to be less than the gate drive voltage of the MOSFET, i.e. the difference between the gate-source voltage applied across it, which is in fact formed by the input voltage, and its threshold voltage. The MOSFET is consequently being operated in strong inversion, so that it lies in the resistive region of the output characteristic, also referred to as the xe2x80x9clinear regionxe2x80x9d or xe2x80x9cactive regionxe2x80x9d.
In the resistive region, the drain current is proportional to the drain-source voltage to a good approximation. Because of this proportionality, the channel of the MOSFET can thus be assigned a resistance or conductance. This conductance is itself proportional to the gate drive voltage. An increase in the input voltage, and hence the gate drive voltage, therefore effects a proportional increase in the conductance and hence also in the drain current. Since the drain current programs the first current mirror, the current flowing through the second transistor, which in fact forms the output current of the voltage-current converter, is consequently also increased proportionally, but in accordance with the factor, the output current remains at just one tenth of the current through the first transistor. Thus the output current is proportional to the input voltage, as is expected of course from a voltage-current converter.
Preferably, provision is made for the first current mirror to contain a third transistor that is connected to ground, where the current flowing through it, rather than the current flowing through the second transistor, now constitutes the output current of the voltage-current converter. This third transistor therefore acts as an output transistor, so that the input voltage is not loaded by the output current. This achieves a higher input resistance for the voltage-current converter. In addition, using this third transistor, the output current can be scaled to the required order of magnitude independently of the second transistor.
Preferably, in the second current mirror, the current flowing through the first transistor is equal to the current flowing through the second transistor. This simplifies the design of the circuit and the layout.
Preferably, in the first current mirror, the first transistor and the second transistor are operated in weak inversion. As a result, the drain-source voltage remains constant over a large range of several decades, improving the accuracy of the voltage-current converter.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a voltage-current converter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.